The present invention is directed toward an electroslag melting system and more particularly toward a system which utilizes an air tight sleeve to provide high integrity atmosphere control during electroslag melting and which also functions as a return conductor.
The electroslag melting process was first invented, developed and put into full production by R. F. Hopkins in the United States during the period between 1930 and 1960. This process employs a consumable electrode which is immersed in a pool of molten slag supported at the top of the resultant solidifying ingot enclosed within a cold-walled mold or crucible.
Alternating (or sometimes direct) current flows down the consumable electrode through the slag, down the ingot and back to the power supply. Preferably, the current flows back to the power supply in a coaxial manner to the top of the crucible such as shown in copending application Ser. No. 616,365, filed Sept. 24, 1975 now U.S. Pat. No. 4,032,705. This current, normally in the range of 1,000 amps per inch of ingot diameter, drops from fifteen to forty volts across the slag (or flux) pool thereby producing hundreds of kilowatts of melting power which consumes the tip of the electrode.
As a result of the foregoing, molten metal droplets form on the immersed electrode tip, detach themselves and fall through the molten flux pool to the ingot which is forming therebelow. As the metal droplets pass through the flux pool, they undergo chemical refinement. Progressive solidification of the ingot formed by this method leads to the physical isotropy and high yield associated with all consumable electrode processes.
Melting rates in the electroslag process are determined by the solidification characteristics of each alloy. However, as an average and for illustration purposes only, such rates are approximately 25 pounds per hour per inch of ingot diameter. Thus, a 24 inch diameter ingot of alloy steel might have an average melt rate of 600 pounds per hour. If this ingot has a typical height of 96 inches, its weight will be six tons and total melting time will, therefore, be approximately twenty hours.
As is known in the art, motion of the head of the electrode is the difference between the rate of burn-off of the electrode tip and the rate of build-up of the ingot being formed therebelow. In the preceding example, a twenty inch diameter electrode would typically be used and its consumable length would need to be greater than the ingot length in the inverse ratio of the squares of their diameter, assuming of course, full density for both.
During melting, gases which are deleterious to the finished ingot are capable of being transported across the molten slag and into the molten melter pool at the head of the ingot. This is particularly true of hydrogen gas. Thus, in alloys which are sensitive to the gas content and in particular to those which are subject to hydrogen embrittlement, it is most desirable to control the nature of the atmosphere above the molten slag.
This desirability becomes a virtual necessity as ingot diameters increase to approximately one meter and above for the following reasons. With this size ingot it is more difficult for hydrogen to migrate to the external surface of the ingot thereby removing the possiblity of hydrogen cracking. In addition, larger electroslag ingots are primarily required in the field of medium to heavy forgings and most forging grades are susceptible to hydrogen embrittlement.
In the past many different methods have been employed to achieve partial atmosphere control above the surface of the molten slag. Techniques such as hooding and flushing and the reliance upon the fact that argon is heavier than air to flood the space above the molten slag pool have been tried. However, none of these methods have been more than partially successful. This is true partially because of the very strong convection currents above the molten slag and partially because only a small amount of moist air brought in contact with the molten flux is sufficient to permit hydrogen to pass through the flux and into the solidifying ingot.
It is also true that most electroslag furnaces are of generally open geometry for a number of operating reasons. Therefore, the feasibility of high integrity atmosphere control in a production electroslag furnace has not been recognized until this time.
Electroslag melting has, in the past, been done in vacuum arc furnaces which means that full control of the atmosphere was automatically available to the melter. However, A.C. electroslag melting, which has become generally adopted because of improved refining characteristics cannot be conducted in a standard vacuum arc furnace because of eddy current heating and poor power factors.
In prior copending application Ser. No. 773,334 filed Mar. 1, 1977, a system was disclosed which provides high integrity atmosphere control of electroslag melting processes. As disclosed therein, this is accomplished by providing an air tight sleeve between the top of the crucible and the bottom of the furnace head. While this technique more than satisfactorily accomplishes the desired atmosphere control, it may create problems with the physical placement of the return conductors which normally are located within the space between the crucible and the furnace head. This problem becomes more apparent as the number and size of the return conductors increases with larger diameter furnaces.